1. Field of the Invention
The present invention relates to a printing method, and more particularly, to a printing method for interpolating gray levels.
2. Description of the Prior Art
Photo printers are different from general printers. The major difference is that the photo printer can print out an image such as a photo picture on paper with high picture quality. Please refer to FIG. 1 and FIG. 2. FIG. 1 is a diagram of a prior art photo printer 10. FIG. 2 is a simplified exploded view of the photo printer 10 shown in FIG. 1. As shown in FIG. 1, the photo printer 10 has a ribbon 14, a thermal print head 12, a ribbon driver 18, and a roller set 20. The ribbon 14 has a plurality of sectors, and each sector is used for storing one kind of different color dyes. The thermal print head 12 is fixed inside the photo printer 10 for heating the color dyes so that the color dyes are transferred onto a photo paper 16. The ribbon driver 18 is used for moving the ribbon 14 back and forth so that the thermal print head 12 can transfer a specific color dye stored on the ribbon 14 onto the corresponding photo paper 16. The roller set 20 is used for holding the photo paper 16 and moving the photo paper 16 along a predetermined direction. Therefore, the fixed thermal print head 12 is capable of printing a color image on the photo paper 16.
As shown in FIG. 2, the thermal print head 12 has a plurality of heaters 22 that are arranged linearly and spaced equally for heating the ribbon 14. The color dye stored on the ribbon 14 is heated, and is transferred onto the photo paper 16. When the thermal print head 12 starts printing images, each heater 22 positioned on the thermal print head 12 will heat the ribbon 14 so that a plurality of corresponding pixels X1 will form a line image Y1. Then, the photo paper 16 driven by the roller set 20 is moved along the predetermined direction according to a predetermined speed. Therefore, another line image Y2 is printed on the same photo paper 16 next to the line image Y1. Accordingly, a plurality of line images are successfully printed on the same photo paper 16 to complete the printing operation.
As mentioned above, the total number of heaters 22 positioned on the thermal print head 12 determines the corresponding number of the pixels X1 of each line image printed on the photo paper 16. Moreover, the color concentration, that is, the gray level of each pixel X1 printed on the photo paper 16 is determined by the corresponding heater 22 with a specific duration of each heating operation and a total number of heating cycles.
Please refer to FIG. 3A and FIG. 3B. FIG. 3A is a diagram of gray levels and a corresponding driving signal 30 according to the photo printer 10 shown in FIG. 1. FIG. 3B is a diagram of a binary data sequence of the driving signal 30 shown in FIG. 3a. As shown in FIG. 3A and FIG. 3B, before the thermal print head 12 of the photo printer 10 starts printing images onto the photo paper 16, all of the heaters 22 positioned on the thermal print head 12 are activated during a predetermined period Tp so that each heater 22 will first approach a predetermined printing temperature. The above-mentioned procedure is called a preheating operation. In addition, the driving signal having a pulse with a binary value xe2x80x9c1xe2x80x9d will activate the corresponding heater 22, and the driving signal corresponding to a binary value xe2x80x9c0xe2x80x9d will not activate the heater 22. Next, the photo printer 10 will continuously activate the same heater 22 according to the corresponding gray level of the pixel X1. In other words, each heater 22 positioned on the thermal print head 12 is activated repeatedly according to the desired gray level of the corresponding pixel. The overall heating operation of the heater 22 is represented by a driving signal 30 and its corresponding binary values. Each duration Tu of a pulse 32 is a heating time unit for activating the heater 22. In addition, the energy generated by the heater 22 onto the corresponding pixel X1 during the duration Tu of each pulse 32 is nearly identical. That is, the quantity of color dyes transferred onto the photo paper 16 during the fixed duration Tu is almost identical. The reason why the quantity of color dyes is almost identical is because of a thermal accumulation effect. It is well known that the thermal accumulation effect is adjusted according to a prior art control method so that the quantity of color dyes is controlled with acceptable inaccuracy. A lengthy description of the prior art control method is skipped for brevity.
The heater 22 of the photo printer 10 can produce 256 (0xcx9c255) gray levels to print the corresponding pixel X1 with an appropriate gray level. A gray level corresponding to a lightest color concentration is equal to 0, and a gray level corresponding to a darkest color concentration is equal to 255. In other words, when the pixel X1 acquires a corresponding gray level equaling N, which is an integer between 0 and 255, the corresponding heater 22 has to be successively activated N times. Therefore, N pulses 32 of the driving signal 30 are generated repeatedly. That is, N binary xe2x80x9c1xe2x80x9d values are inputted to the heater 22 continuously. Please note that the photo paper 16 is printed one line at a time. Because each pixel X1 positioned on the same line may have different gray levels, each heater 22 has to wait for 255 durations Tu so that the thermal print head 12 can then print the next line image. That is, one heater 22 finishes printing a corresponding pixel X1 with a smaller gray level within a short time. But, another heater 22 printing a corresponding pixel X1 with a greater gray level may take a long time. When the total number of different gray levels is doubled, each heater 22 has to wait for 511 durations Tu. Therefore, if the number of different gray levels is increased, each heater 22 has to operate for a longer period to complete printing one line image. That is, if the color resolution is improved, the execution time is longer. The printing efficiency, therefore, is greatly deteriorated.
It is therefore a primary objective of the claimed invention to provide a printing method for interpolating gray levels of a thermal print head to solve the above mentioned problem.
Briefly, the claimed invention provides a printing method using a thermal print head having a plurality of heaters linearly arranged and equally spaced for heating a dye and transferring the dye onto an object, thereby forming a plurality of pixels corresponding to the heaters on the object. A color of each pixel is determined by a gray level. Each gray level comprises a first portion and a second portion. When controlling the heater to generate a pixel of a predetermined gray level, the printing method comprises activating a heater for a number of cycles corresponding to the first portion of the predetermined gray level, thereby transferring the dye onto the object in a position corresponding to the heater.The first portion is larger than or equal to zero. Each cycle lasts a substantially equal amount of time. Each activation of the heater within a cycle lasts a substantially equal amount of time, and quantity of the dye transferred onto the object is substantially equal for each activation of the heater. The printing method further comprises deactivating the heater for a first predetermined number of cycles corresponding to the second portion of the predetermined gray level, then activating the heater a second predetermined number of cycles corresponding to the second portion of the predetermined gray level. Both the first predetermined number and the second predetermined number are integers larger than or equal to 1. A total quantity of the dye transferred onto the object in printing the second portion of the predetermined gray level is less than the quantity of dye transferred onto the object during each cycle of printing in first portion of the predetermined gray level.
It is an advantage of the claimed invention that the claimed printing method can improve the output picture quality and the printing efficiency of the photo printer by interpolating gray levels based on the thermal accumulation effect.
These and other objectives of the claimed invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment which is illustrated in the various figures and drawings.